(a) Technical Field
The present disclosure relates to a surface-treated cathode active material and a lithium secondary battery using the same. More particularly, the present disclosure relates to a surface-treated cathode active material useful for manufacturing a lithium secondary battery having excellent output characteristics by performing a double coating by primarily coating metal oxide on a surface of a cathode active material for the lithium secondary battery and secondarily coating a conductive polymerized copolymer having both ion conductivity and electron conductivity thereon to enhance electrochemical properties and thermal stability of the cathode active material, and the lithium secondary battery using the same.
(b) Background Art
A lithium secondary battery is manufactured by using a material capable of intercalating and deintercalating lithium ions as a negative electrode and a positive electrode and intercalating an organic electrolytic solution or a polymer electrolyte which enables lithium ions to move between the negative electrode and the positive electrode, and stores electrical energy by means of redox reactions according to the intercalation and deintercalation of lithium ions in the positive electrode and the negative electrode.
In order to enhance electrochemical properties and safety of the lithium secondary battery as described above, studies on surface treatments of the cathode active material of the lithium secondary battery have been actively performed.
In a case of the cathode active material for the lithium secondary battery, dissolution of Li by HF produced in the electrolytic solution may be prevented by using metal oxide (Al2O3, ZrO2 and La2O3), metal phosphorus oxide (AlPO4 and LiCoPO4), carbon, halogen gas, metal hydroxide, a conductive polymer and the like to reduce a direct reaction with an electrolyte through a positive electrode surface coating of several nm, and the stability of a crystal structure may be secured by suppressing elution of various transition metals.
However, there is a limitation in that a drawback occurs, when the movement of lithium ions and electronic conduction are interrupted in the case where a coating material, such as metal oxide which is a non-conductor, is used so that the mobility of lithium ions and electrons deteriorates. Further, when coating with an inorganic particle alone, it is difficult to secure uniformity of the coating. In particular, it is insufficient to secure the structural stability of the cathode active material due to non-uniformity of the coating while a side reaction with the electrolytic solution occurs.
In a case of a polymer coating, it is possible to perform coating with an organic material as the related art in terms of securing the uniformity of the coating, however due to thermal instability of the organic material and a too large thickness of the coating layer, ions may not be smoothly transfer, so that performance deteriorates.
Accordingly, depending on the particular inorganic material or polymer, the cathode active material has a limitation which may not be overcome in improving physical properties by only the surface modification.
For this reason, as a cathode active material surface modification technology for improving high capacity, high output, and service life characteristics of the lithium secondary battery, a technology of coating manganese oxide with a conductive polymer or obtaining a coating by mixing two materials have been studied, however there is only an intention to obtain improvement in performance by simply increasing the content of a conductive material.
As an example of the related art, Korean Patent Application Publication No. 2007-8115 proposes a cathode active material sequentially including a first covering layer of oxide on a surface of a lithium transition metal oxide particle, and a second covering layer of a conductive material on the first covering layer as a technology having a double coating structure. However, the material used in the second covering layer is a pure electron conductive material, and thus the movement of lithium ions is not smooth during a charging and discharging process.
Korean Patent Application Publication No. 2011-23067 proposes a cathode active material for a lithium secondary battery including a lithium metal oxide secondary particle core, a first shell formed by coating the surface of a secondary particle core portion with a plurality of barium titanate particles and a plurality of metal oxide particles, and a second shell formed by coating a surface of the first shell with a plurality of olivine-type lithium iron phosphate oxide particles and a plurality of conductive material particles. Furthermore, Korean Patent Application Publication No. 2007-16431 proposes an active material for a lithium secondary battery, which has a core material and a surface treatment layer which is formed on a surface of a core material and includes an inorganic particulate having a nano-size and conductive polymer.
However, according to the technologies of the surface coating-treated cathode active material as described above, although surface modification effects are improved as compared to the existing technologies, conductivity and ion transfer effects are not fully exhibited, and effects of the improvement in performance such as electrochemical properties and thermal stability are slight.
Japanese Patent Application Publication No. 2005-524936 proposes an electrode manufactured while a conductive material and an ion conducting polymer are extruded into an active material including metal oxide through an extruder. However, effects of the improvement in performance such as electrochemical properties and thermal stability are not very good because the structure of the active material is unstable and non-uniform.
In addition to the technologies described above, there are several technologies using a conductive polymer and a metal powder, such as a technology in which a cathode active material is coated with a polymer having both ion conductivity and electric conductivity and a polymer film to which a conductive metal powder is added. However, it does not sufficiently resolve the existing problems in improving physical properties, such as maintaining a balance between conductivity and ion transfer effects.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.